The present invention relates generally to improvements in thermometers and, more particularly, to electronic thermometers for more rapidly obtaining accurate temperature measurements.
It is common practice in the medical field to determine the body temperature of a patient by means of a temperature sensitive device that not only measures the temperature but also displays that temperature. Such temperature measurements are taken routinely in hospitals and in doctors' offices. One such device is a glass bulb thermometer incorporating a heat responsive mercury column that expands and contracts adjacent a calibrated temperature scale. Typically, the glass thermometer is inserted into the patient, allowed to remain inserted for a sufficient time interval to enable the temperature of the thermometer to stabilize at the body temperature of the patient, and subsequently removed for reading by medical personnel. This time interval is usually on the order of 3 to 8 minutes.
The conventional temperature measurement procedure using a glass bulb thermometer or the like is prone to a number of significant deficiencies. Temperature measurement is rather slow and, for patients who cannot be relied upon (by virtue of age or infirmity) to properly retain the thermometer for the necessary period of insertion in the body, may necessitate the physical presence of medical personnel during the relatively long measurement cycle, thus diverting their attention from other duties. Furthermore, glass bulb thermometers are not as easy to read and, hence, measurements are more susceptible to human error, particularly when the reading is made under poor lighting conditions or when read by harried personnel.
Various attempts have been made to minimize or eliminate these deficiencies of the glass bulb thermometer by using temperature sensing probes that are designed to operate in conjunction with direct-reading electrical thermometer instrumentation. In one such approach, an electric temperature sensitive device, such as a thermistor, is mounted at the end of a probe and inserted into the patient. The change in voltage or current of the device, depending on the particular implementation, is monitored and when that output signal stabilizes, a temperature is displayed in digital format. This is commonly referred to as the "direct reading" approach and it reduces the possibility of error by misreading the measured temperature. This approach has provided a significant contribution to the technology of temperature measurement.
An inherent characteristic of electronic thermometers is that they do not instantaneously measure the temperature of the site to which they are applied. It may take a substantial period of time before the temperature sensitive device stabilizes at the temperature of the site and the temperature indicated by the thermometer is representative of the actual temperature of the body or site measured. This lag is caused by the various components of the measurement system that impede heat flow from the surface of the body or site to the temperature sensor. Some of the components are the sensor tip, the tissue of the body, and any hygienic covering applied to the sensor tip to prevent contamination between measurement subjects.
One attempt to shorten the amount of time required to obtain a temperature reading of a subject involves the use of a temperature sensitive electronic probe coupled with prediction or estimation circuitry or programming to provide a digital display of the patient's temperature before the probe has reached equilibrium with the patient. With this approach, assuming the patient's temperature is not significantly changing during the measurement period or cycle, the temperature that will prevail upon thermal stabilization of the electronic thermometer with the patient is predicted from measured temperatures and is displayed before thermal stabilization is attained. Typically, prediction of temperature is performed by monitoring the measured temperature over a period of time as well as the rate of change thereof, and processing these two variables to predict the patient's temperature.
With an electronic thermometer that operates by predicting the final, steady state temperature, an advantage is that the temperature measurement is completed before thermal stabilization is attained, thereby reducing the time required for measurement. This would lessen the risk that the patient would not hold the probe in the correct position for the entire measurement period and requires less time of the attending medical personnel. Another advantage is that because body temperature is dynamic and may significantly change during the five minute interval typically associated with traditional mercury glass thermometer measurements, a rapid determination offers more timely diagnostic information. In addition, the accuracy with which the temperature is predicted improves markedly as the processing and analysis of the data are more accurately performed. This approach has also contributed significantly to the advancement of temperature measurement technology.
Electronic thermometers using predictive-type processing and temperature determination may include a thermistor as a temperature-responsive transducer. The thermistor approaches its final steady state temperature asymptotically with the last increments of temperature change occurring very slowly, whereas the major portion of the temperature change occurs relatively rapidly. Prior attempts have been made to monitor that initial, more rapid temperature change, extract data from that change, and predict the final temperature at which the thermistor will stabilize and therefore, determine the actual temperature of the tissue that is contacting the thermistor long before the thermistor actually stabilizes at the tissue temperature.
A prior approach used to more rapidly predict the tissue temperature prior to the thermistor reaching equilibrium with that tissue is the sampling of data points of the thermistor early in its response and from those data points, predicting a curve shape of the thermistor's response. From that curve shape, an asymptote of that curve and thus the stabilization, or steady state, temperature can be predicted. To illustrate these concepts through an example of a simpler system, consider the heat transfer physics associated with two bodies of unequal temperature, one having a large thermal mass and the other having a small thermal mass, placed in contact with each other at time t=0. As time progresses, the temperature of the small thermal mass and the large thermal mass equilibrate to a temperature referred to as the stabilization temperature. The equation describing this process is as follows: EQU T(t)=T.sub.R +(T.sub.F -T.sub.R)(1-e.sup.-(t/.tau.))=T.sub.F -(T.sub.F -T.sub.R)e.sup.-(t/.tau. (Eq. 1)
where: T(t) is the temperature of the smaller body as a function of time,
T.sub.R is the initial temperature of the smaller body, PA1 T.sub.F is the actual, steady state temperature of the system, PA1 t is time, and PA1 .tau. is the time constant of the system. PA1 T'=first derivative PA1 T"=second derivative.
From this relationship, when T is known at two points in time, for example T.sub.1 at time t.sub.1 and T.sub.2 at time t.sub.2, the stabilization temperature T.sub.F can be predicted through application of Equation 2 below. ##EQU1##
Further, for a simple first order heat transfer system of the type described by Equation 1, it can be shown that the natural logarithm of the first time derivative of the temperature is a straight line with slope equal to -1/.tau. as follows: ##EQU2##
and also: EQU T.sub.F =T(t)+.tau.T'(t) (Eq. 3.2)
where: ##EQU3##
where: K=a constant dependent upon T.sub.R, T.sub.F, and .tau.,
Prior techniques have attempted to apply these simple first order relationships through the use of thermistor time constants established by the thermistor manufacturer. However, these techniques have failed to recognize that the temperature response curve cannot be modeled as first order and is to a great extent affected by factors not reflected by the thermistor's time constant. When the thermometer is placed in contact with body tissue, such as a person's mouth for example, the response curve depends on the physical placement of the probe in relation to that tissue, on the heat transfer characteristics of the particular tissue, and on the hygienic cover that separates the probe from the tissue. All of these factors contribute to the heat flow characteristics of the measurement system and they are not represented in the factory-supplied time constant of the thermistor alone. Moreover, the factors described above impede the flow of heat in series and with different resistance characteristics, thus causing an overall time response behavior that is not that of a first order system.
While electronic thermometers and prior predictive techniques have advanced the art of electronic thermometry, a need still exists for an electronic thermometer that can predict a stabilization temperature at an early stage of the measurement process and thereby shorten the amount of time taken to obtain a final temperature reading. Additionally, a need exists for a thermometer having an algorithm that can be computed in readily available, relatively simple, relatively inexpensive circuitry or processors. The invention fulfills these needs and others.